1. Field of the Invention
The present invention relates to a process for producing 2-methyl-1,4-naphthoquinone by oxidation of 2-methylnaphthalene in liquid phase.
2-Methyl-1,4-naphthoquinone, sometimes called menadione or vitamin K3, has anti-hemorrhagic activity. Many derivatives of 2-methyl-1,4-naphthoquinone including vitamins K1, K2, K4, sodium bisulfite adduct of vitamin K3 and dimethylpyrimidinol bisulfite adduct of vitamin K3 have been employed as medicines for human and additives for animal feed and are thus useful.
2. Related Art of the Invention
Typical of known industrial processes of producing 2-methyl-1,4-naphthoquinone is a process wherein 2-methylnaphthalene is oxidized by use of chromic anhydride or bichromates such as sodium bichromate. However, the ecological problem caused by chromium compounds has made it difficult to produce 2-methyl-1,4-naphthoquinone by the process in these days.
On the other hand, many attempts have been made for oxidation processes which don't use any chromium compound. One of such attempts is a process wherein 2-methylnaphthalene is oxidized, in vapor phase, with oxygen by use of a vanadium-based catalyst (see Japanese Patent Laid-open No. Hei 6-9485). In order to attain high selectivity, the conversion has to be suppressed to a very low level in this process.
For an oxidation process in liquid phase, there is a process using cerium compounds (see U.S. Pat. No. 4,840,749). According to this process, the intended compound can be obtained at a relatively high yield. However, because a stoichiometric amount of a cerium compound is used, it is required to regenerate the cerium compound, for example, by electrolysis.
Other known processes include processes wherein 2-methylnaphthalene in liquid phase is oxidized by use of hydrogen peroxide or organic peracids. The reaction procedures in the processes include a procedure wherein oxidation is conducted in a solvent, such as acetic acid, without use of any catalyst (see Japanese Patent Publication No. Sho 59-53252) and a procedure wherein acids are used as a catalyst (see Japanese Patent Laid-open No. Sho 53-50147). In either case, the yield is as low as less than 40%. An attempt has been made to use, as a catalyst, ion-exchange resins deposited with palladium (see Japanese Patent Laid-open No. Sho 61-227548). In this process, a high yield exceeding 50% is attained. However, a great amount of ion-exchange resins is used relative to the reactant. From the standpoint of the cost for the ion-exchange resin and the prevention of contamination by impurities from the ion-exchange resin, it is necessary to impart very high durability to the ion-exchange resin. Test results are reported in Chem. Pharm. Bull., 34(2), 445-449 (1986), where oxidation of 2-methylnaphthalene with hydrogen peroxide and the recycled palladium ion-exchange resin catalyst in an acetic acid is conducted repeatedly. According to the report, the yield is 60.2% for the first cycle and is 50.5% for the fourth cycle, thus lowering by about 16%. The follow-up test made by us revealed that the catalytic activity was lost after 5 to 6 maximum repetitions in use. This means that the cost for the catalyst becomes very high, making it difficult to industrially adopt this process from an economical point of view. Where ion-exchange resins are used in batchwise reactors, the ion-exchange resin is stirred along with a reaction solution and, thus, should have appropriate physical strength. On the other hand, when ion-exchange resins are employed in a continuous reactor, removal of the heat is very difficult, since the reaction is highly exothermic. To attain a satisfactory conversion of 2-methylnaphthalene according to this process, hydrogen peroxide has to be used in amounts of not less than 6 times by mole relative to the 2-methylnaphthalene, that is twice the theoretical amount. This will not only cause a high ratio of the cost of hydrogen peroxide to the total cost of starting chemicals, but also bring about generation of oxygen gas caused by the decomposition of excessive hydrogen peroxide and/or accumulation of the peroxide in the reaction system, with the great possibility of ignition and explosion during the course of running operations.